Problem: The scale on a map is 9cm : 3km. If the distance between two cities is 18km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 9cm on the map corresponds to an actual distance of 3km. An actual distance of 18km is the same as 6 $\cdot$ 3km. The distance between the two cities on the map is 6 $\cdot$ 9cm, or 54cm.